The present invention relates to a recorded data playback apparatus for reproduction of data from a recording disc by photo-electric sensing means, e.g. of the type in which a light beam such as a laser light beam is directed onto recording tracks of the disc and the resultant reflected light is sensed. With such an apparatus, it is necessary that this data sensing light beam be held precisely positioned with respect to the recording tracks, i.e. that the light beam precisely follows a single recording track. This is because, if any light from the data sensing light beam should fall upon an adjacent recording track, then the resultant reflected light will be sensed and will produce crosstalk. It is therefore necessary to maintain a precisely mutually perpendicular relationship between the data sensing light beam and the recording disc surface. This is generally accomplished by utilizing a servo system, sometimes referred to as a tilt servo system, which is coupled to the pick-up of the apparatus and acts to detect and compensate for any deviations from an accurate mutually perpendicular relationship between the data sensing light beam and the recording disc. sensing means mounted on the pick-up (i.e. the unit which emits the data sensing light beam and receives and transduces the resultant reflected light from the recording disc into a data signal) of the recorded data playback apparatus for detecting a deviation from the mutually perpendicular relationship, producing a signal which varies in accordance with such a deviation, a servo amplifier which receives the latter signal, and a servo motor which is driven by an output signal from the servo amplifier and is coupled to the pick-up and acts to move the pick-up such as to vary the degree of tilt of the data sensing light beam with respect to the surface plane of the recording disc such as to compensate for the deviation from the mutually perpendicular relationship between the data sensing light beam and the recording disc surface. The photo-sensing means referred to above generally comprise a photo-emissive element for directing light towards the recording disc surface, and photo-receptive elements which are positioned such as to detect light reflected from the recording disc surface resulting from that emitted by the latter photo-emissive element and to thereby produce output signals whose relationship varies in accordance with the degree of deviation from the mutually perpendicular relationship of the data sensing light beam and the recording disc. These output signals are sensed by means (e.g. a differential amplifier) which drive the servo amplifier.
Such a tilt servo system can provide very accurate maintenance of the mutually perpendicular relationship between the data sensing light beam and the recording disc. However if the recorded data playback apparatus is set in operation with no recording disc mounted on the turntable of the apparatus, or if a recording disc is removed from the apparatus while it is in operation, then the amount of light which is reflected back to the photo-receptive elements of the tilt servo system will fall considerably from that reflected when a recording disc is mounted. As a result, the "tracking lock" servo loop status will be lost, and runaway of the tilt servo system may result. This is a serious problem with prior art types of tilt servo system applied to such a recording disc playback apparatus.